As shown in FIG. 2, the control mechanism of revolving speed of an electric tool is generally composed of a gear set seat 1, a gear set A 20, a gear set B 21, a gear set C 22, a fixing toothed sleeve 28, a sliding toothed ring 24, etc. In addition, the control mechanism is provided with a push button switch in conjunction with a connection rod action mechanism. In operation, the push button switch is moved back and forth so as to cause the connection rod to actuate the sliding toothed ring 24 to move back and forth to cause the outer ring teeth 240 to engage or disengage the ring teeth 230 located on the inner wall of the fixing toothed sleeve 23. When the sliding toothed ring 24 is moved forward to engage the fixing toothed sleeve 23, the gear set B 21 which is engaged with the sliding toothed sleeve 24 is activated to reduce the revolving speed of the hand tool. On the other hand, when the sliding toothed ring 24 is caused to move rearwards so as to disengage the fixing toothed sleeve 23, the sliding toothed ring 24 and the gear set B 21 are idled so that the gear set B 21 does not act to reduce the revolving speed of the electric tool, thereby unabling the electric tool to revolve at a high speed.
Such a control mechanism of revolving speed of the electric tool as described above has inherently two shortcomings, which are described hereinafter.
When an electric tool, which is operating at a high speed, is switched to operate at a lower speed, the outer ring teeth 240 of the sliding toothed ring 24 are not so located as to be ready to engage the ring teeth 230 of the fixing toothed sleeve 23. As a result, the push button switch can not be directly switched into a low speed position. Therefore, a tool operator is required to press continuously the push-button switch until such time when the sliding toothed ring 24 is actuated to engage the fixing toothed sleeve 23. It is therefore readily apparent that it is undesirable to employ the push-button switch in conjunction with the connection rod mechanisim. In addition, the sliding toothed ring 24 is susceptible to tripping caused by a reaction force brought about by the electric tool which is operated under a torsional stress that is slightly greater than normal. Such a tripping incident of the sliding toothed ring 24 as described above can result in the idling of the electric tool.